1. Field
The present invention relates to transmission-path-type specifying apparatuses and methods and, in particular, to a transmission-path-type specifying apparatus and method of specifying the type of a transmission path for transmitting an optical signal subjected to wavelength division multiplexing (WDM).
2. Description of the Related Art
In recent years, a communication technology has attracted attention in which a plurality of optical signals with different wavelengths are subjected to Wavelength Division Multiplexing (WDM) for transmission through an optical fiber. In a transmission using this WDM technique, regarding an optical pulse output to the optical fiber, when optical output power increases, an interference occurs due to a non-linear effect to distort a transmission waveform. For this reason, the maximum optical output power that can be output to the optical fiber is restricted. This maximum optical output value differs depending on the type of optical fiber. Thus, specifying the type of optical fiber is an extremely important factor in transmission using WDM.
Meanwhile, for transmission using WDM, a technology has been disclosed in which a transmitting apparatus of an optical transmission system outputs Amplified Spontaneous Emission (ASE) light, which is generated by supplying pumped light for amplifying an optical signal to an optical amplifier, to an optical fiber and then a receiving apparatus of the optical transmission system extracts from the output ASE light a plurality of different wavelengths to calculate a dispersion amount indicative of a propagation-delay-time difference among these wavelengths, thereby controlling a wavelength dispersion compensation amount so that the propagation delay time is 0 (refer to Japanese Patent Application Laid-open No. 2005-286906).
It is known that, by using this dispersion amount, the type of optical fiber is specified. In the following, by using FIGS. 27 and 28, a conventional method of specifying the type of an optical fiber is explained. FIG. 27 is a drawing of a conventional method of specifying the type of an optical fiber. In the method of specifying the type of an optical fiber depicted in FIG. 27, an Optical Time Domain Reflectometer (OTDR) is incorporated in an optical fiber. The OTDR outputs to the optical fiber a pulse superposed on light of two different types of wavelength (λ1, λ2), thereby measuring a dispersion amount (ps/nm/km) per unit length of the optical fiber with the use of a reflected pulse of the transmission pulse. Specifically, two laser diodes serving as wavelength light sources emit a pulse superposed by a pulse modulating unit onto pumped light of two different types of wavelength for amplification. Then, these two types of pulse pass through an optical coupler (CPL) and then an optical Consumer Infrared (CIR) for output to the optical fiber, as depicted in an upper-right portion of FIG. 27. Inside the optical fiber, a reflected pulse of the transmission pulse returns to the OTDR, as depicted in a lower-right portion of FIG. 27, with a waveform being attenuated due to part of the transmission pulse scattering in a direction reverse to a traveling direction. In the OTDR, the reflected pulse passes through the optical CIR to be demultiplexed at a wavelength filter, be converted by two photodiodes serving as receiving elements from an optical pulse to an electrical signal, and then be output to a level sampling unit. The level sampling unit measures a delay time (T) from the time when the transmission pulse is transmitted to the optical fiber to the time when it returns, an attenuation amount (X) corresponding to the delay time, and a loss factor (α). Then, the level sampling unit calculates a length (L) of the optical fiber by using equations depicted in a lower right portion of FIG. 27 from the attenuation amount (X) and the loss factor (α) to calculate a dispersion amount (D) per unit length of the optical fiber from the calculated length of the optical fiber and the delay time (T).
Here, dispersion characteristics of various optical fibers are explained by using FIG. 28. FIG. 28 depicts a dispersion amount (ps/nm/km) per unit length of the optical fiber, an inclination, and a maximum output power that can be output to the optical fiber, for each of types of optical fiber corresponding to a plurality of wavelengths that belong to a conventional band (C-BAND) and a long band (L-BAND), which are wavelength bands for WDM. In FIG. 28, the inclination represents a value obtained by dividing an increase in dispersion amount per unit length of the optical fiber by an increase in wavelength. A relation between the dispersion amount per unit length of the optical fiber and the wavelength shows linear approximation unique to the type of optical fiber found from the inclination for each type of optical fiber. Therefore, if the dispersion amount (D) per unit length of the optical fiber and an average wavelength between λ1 and λ2 are known, the optical fiber is specified with linear approximation unique to the type of optical fiber. Furthermore, maximum optical output power that can be output to the optical fiber is specified.
However, the method of specifying the type of an optical fiber explained above has a problem of high cost. That is, the OTDR includes at least two laser diodes serving as wavelength light sources and two photodiodes serving as receiving elements, which are generally expensive, resulting in high cost in the method of specifying the type of an optical fiber. In particular, specifying the type of optical fiber is performed only once at the time of start-up before operation of the optical transmission system and it is not rational to allow high cost for such a process as performed only once. Therefore, there is a need for specifying the type of optical fiber not at high cost.
Moreover, when dispersion amounts per unit length of a plurality of optical fibers are analogous to each other, the type of optical fiber cannot be specified even if the OTDR is used to specify the type of optical fiber.
Still further, in Japanese Patent Application Laid-open No. 2005-286906 mentioned above, the receiving apparatus of the optical transmission system calculates a dispersion amount, but does not calculate a length of the optical fiber. Therefore, the dispersion amount (D) per unit length of the optical fiber cannot be calculated from the dispersion amount, thereby making it impossible to specify the type of optical fiber. Therefore, even if this optical transmission system specifies the type of optical fiber by using the OTDR mentioned above, high cost is still required.